In recent years, a reduction in the size of endoscopes and an improvement in the image quality thereof have progressed from the standpoint of relieving the burden on patients, improvements in diagnostic accuracy, etc. Thus, high-pixel-count and compact imaging devices have been developed as endoscope imaging devices (for example, CCD, CMOS, etc.). In such imaging devices, in order to increase the number of pixels while maintaining or reducing the size thereof, each pixel and the pixel pitch are reduced. With these reductions, it is also necessary to reduce the Fno. of an endoscope objective optical system in order to prevent image degradation caused by diffraction.
Incidentally, in an objective optical system, the observation depth is narrowed when the Fno. is reduced. Thus, various endoscope objective optical systems having a focusing function in order to obtain a desired observation depth have been proposed (for example, PTL 1 to PTL 6). In performing observation by using such an endoscope objective optical system, at the time of transition from normal observation to close observation, it is preferred that only the observation distance be changed, and that the change in the angle of view be small.